1. Field of the Invention
This invention relates to an exhaust manifold system for an automotive transverse V-type engine.
2. Description of the Related Art
Recently, front engine front drive (FF) formula has been widely used in passenger cars with a view to improving roominess, mobility, etc. In such FF type automobiles, an engine is mounted transversely to a vehicle body, i.e., the engine output shaft is oriented in the transverse direction of the vehicle, for structural reasons. In particular, for a multicylinder engine, a V-type engine is used to reduce the engine length, because of the restriction on the space of the engine room, particularly, the vehicle width.
In such transverse V-type engines, an exhaust manifold is provided for each of front and rear banks; the front exhaust manifold for the front bank is mounted to a front portion of the bank, and the rear exhaust manifold for the rear bank is mounted to a rear portion of the bank.
In general, exhaust manifolds for engines are produced by casting metal, or by welding stainless steel pipes, and this is the case with transverse V-type engines. Namely, in conventional transverse V-type engines, exhaust manifolds produced by the same method are used for both the front and rear banks.
When designing the exhaust manifolds for the right and left banks of a longitudinal V-type engine which is mounted to a vehicle body with the engine output shaft oriented in the same direction as the longitudinal direction of the vehicle body, various conditions such as the space, temperature, distance from a catalytic converter for purifying the exhaust gas, do not vary significantly. In the case of the transverse engine, however, since the banks are located in the front and the rear of the engine, respectively, the conditions such as the space, temperature, distance to the catalytic converter greatly vary. For example, the engine is arranged in the engine room as close to the center of the vehicle body as possible, and accordingly, the space for the exhaust manifold of the rear bank is smaller than that for the front exhaust manifold. Further, since the rear exhaust manifold is located at the rear side of the engine, it is only slightly cooled by wind while the vehicle is running, as compared to the front exhaust manifold. Thus, the rear exhaust manifold must be designed such that the exhaust passage therein zigzags, because the rear exhaust manifold is mounted in a small space, and the heatresisting strength is higher than that of the front exhaust manifold, because the rear exhaust manifold is only slightly cooled.
If, however, the front exhaust manifold for the front bank is produced so as to have a heat-resisting strength equal to that of the rear exhaust manifold for the rear bank, then the front exhaust manifold has a higher heat-resisting strength than necessary, which leads to an increase of weight and hence thermal capacity, and increased cost.
Furthermore, although the catalyst should desirably be activated as soon as possible after the start of the engine for the purification of the exhaust gas, the distance of the front exhaust manifold to the catalytic converter is inevitably longer than that of the rear exhaust manifold to the catalytic converter. Therefore, the thermal capacity of the front exhaust manifold is preferably small, so that the exhaust gas from the front bank may not be excessively cooled by the front exhaust manifold.